1. Technical Field
The invention relates to a low cost alternative to the measurement and evaluation of rotary and damping aerodynamic derivatives in a wind tunnel. More particularly, the invention relates to an approach which combines miniature inertial measurement technology with signal processing techniques to allow coupled derivatives and damping terms to be measured. This approach is adapted to existing oscillation techniques for simplifying the measurement approach in lieu of complex heretofore used forced oscillation flexure balances. Single degree of freedom, as well as three degree of freedom damping measurements can be made, and measurement of coupled force and moment coefficients also is possible by modification to the sting adaptor, allowing full advantage of the six degree of freedom capability of the measurement apparatus to be taken.
2. Background Information
In the design of bodies which move through the air, various factors are considered by the designers to determine the effect on the body of various forces and moments exerted on the moving body.
There are six main components of force and moment and three damping terms acting on a model illustrated in FIG. 10 that are of interest to designers in evaluating the flying qualities of such a body. As aircraft configurations have increased in complexity and capability, additional aerodynamic terms are now of significant interest to the designer. These include the coupled force and moment terms referred to as the rotary derivatives (also illustrated in FIG. 10). These thirteen aforementioned components are known by those skilled in the art as vertical force, axial force, side force, pitching moment, yawing moment, rolling moment, pitch damping, yaw damping, roll damping, as well as yaw moment due to roll rate, roll moment due to yaw rate, side force due to roll rate, and side force due to yaw rate. Determining the magnitude of these components acting on the body will determine the flying capability of the body and determine if the desired results can be achieved for most typical configurations. A full description of the aerodynamic terms is in FIG. 10.
Testing for one type of body or an object moving through the air can be for a towed decoy or drogue, such as shown in U.S. Pat. Nos. 4,852,455 and 6,056,236. One method of testing the quality and effects of these forces on a flying object in actual use is by photographic coverage of the object during actual flight. However, this requires that an actual full size body be utilized, and if the tests prove unsatisfactory, require redesign and rebuilding of the actual full size body. Likewise, these tests are expensive and do not provide the accuracy desired.
Recently, tests have been performed on such actual size moving bodies moving in flight by installing a six degree of freedom (DOF) sensor package inside the full size body which measures these various force components and eliminates the need for photographic measurements and provides increasingly reliable test results. However this requires a full size model and expensive in flight tests.
In order to avoid the costly expense of performing in flight tests, whether by photographic means or use of miniature inertia measurement devices in an actual size body, prototypes of the body, either full size or miniature, are tested in a wind tunnel. These models are mounted by various mounting arrangements within the wind tunnel and are provided with various strain gauges which measure the various forces acting on the body to better determine the affects on the body when a full size thereof moves through the air. One such type of six degree of freedom test apparatus is shown in U.S. Pat. No. 5,663,497. Other types of wind tunnel test apparatus and methods of retrieving the desired information are shown in U.S. Publication No. 2003/0000298 and U.S. Pat. No. 5,398,885.
Even though these devices may provide satisfactory inputs, they are relatively expensive to utilize and set up within restricted types of wind tunnels. Furthermore, these experimental methods and wind tunnel testing only measure the six static force and moment derivatives. In order to properly simulate and validate performance from an aerodynamic perspective, and consequently the resultant system design, a full description of the aerodynamic coefficients, to include damping and rotary derivative terms, is required. Inclusion of the coupled force terms is usually of less significance by virtue of the fact that they rarely represent terms of sufficient magnitude to be destabilizing. However, if available, their inclusion will provide a complete description of the aerodynamic performance of the device and so have been included within the design described herein.
The proposed adaptation of this technology in the present invention is designed to measure the time rate of change of the six degree of freedom state vectors of a wind tunnel model. The obtained data is utilized for the determination of the coupled rotary and damping aerodynamic derivatives for the device. The method and apparatus of the present invention eliminates the need for forced oscillation tests as heretofore used in the prior art and which are less desirable due to cost, and wherein the new approach yields improved accuracy over existing free oscillation techniques. The apparatus and method of the present invention is adaptable to forced oscillation driver mechanisms and considerably simplifies the measurement process.
Existing simulation capability for aerodynamic behavior of both towed and freefall or thrusted expendable devices includes the ability to account for not only the standard six force and moment components, but for both the coupled aerodynamic derivatives as well as damping terms. Due to cost of the testing required to accurately capture the dynamic coefficients, inclusion in simulated wind tunnel tests is limited at best, and most times the coefficients utilized are calculated values of the derivatives and not measured results.
Due to the complex nature of the aerodynamic behavior of these devices, the need exists for accurate determination of these coefficients. The configuration of most expendable decoys or drogues which is the preferred embodiment of the present invention, from an aerodynamic perspective, is such that the ratio of roll inertia to pitch and yaw inertia requires the inclusion of coupled terms in the governing differential equations of motion. Furthermore, a further description of the impressed aerodynamic moments, in particular those related to lateral aerodynamics, requires the inclusion of the rotary or cross derivatives.
Current aero-predictive codes provide insight into the trends these values can assume. However, due to the extremely complex nature of the driving aerodynamic phenomena, calculated values often contain sufficient error to corrupt simulation tests. Testing for these derivatives using current techniques is impractical due to the high cost. The proposed approach of the present invention provides a cost effective method and apparatus of measuring these derivatives with sufficient accuracy to meet modeling and simulation requirements in order to provide a true measure of system effectiveness.
Thus, the need exists for an improved method and apparatus for measuring the aerodynamic parameters, including damping and inertially coupled cross derivative terms, through elimination of heretofore extremely expensive forced oscillation balance systems wind tunnel testing by using commercially available miniature inertia sensors (MEMS) and data encoding apparatus which are mounted within the wind tunnel model of the desired device to be tested and transmitted, via either cable or telemetry transmitter, to the external recording system. MEMS is an acronym for “Micro Electro Mechanical Systems” which is the technology utilized in the processing of microchips as applied to miniature device fabrication, such as the micro-inertial sensors of the subject invention. Using MEMS, the gyroscopes and accelerometers of the present invention are made on chips, very inexpensively and accurate, which provides an alternative to the discrete sensors utilized in the system, and results in potential size reduction to be even more compatible with the size restrictions associated with wind tunnel test apparatus.